This invention relates generally to blades that may be useful as wind turbine rotor blades and to rotors and wind turbines utilizing such blades.
Wind turbines have received increased attention as possible environmentally safe and relatively inexpensive alternative energy sources. With such growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted on a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators, generally but not always rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid. Gearless direct drive turbines also exist.
A diameter of wind turbine rotors is sometimes limited by loads subjected to the rotor by wind. For example, peak wind loads during operation of the rotor can cause rotors over a predetermined diameter and/or other components of the wind turbine to fail. Additionally, peak wind loads when the rotor is not operating (i.e., not turning) as well as general fatigue from peak wind loads over time can also cause rotors over a predetermined diameter and/or other components to fail. Passive pitch control of rotor blades (i.e., twist-bend coupling, or TBC) has been used to reduce wind turbine rotor blade loads, and therefore allow larger diameter rotors. Specifically, twisting of sections of the blade reduces an angle of attack of such sections with respect to the wind, thereby reducing peak transient loads that the blade experiences. However, TBC may increase a weight and/or a cost of wind turbine blades that may outweigh any operational gains from a larger diameter rotor. For example, some known rotor blades utilizing TBC are fabricated using carbon fibers angled at between about 5° and about 35° with respect to a pitch axis of the blade in a shell or spar of the blade. However, such known blades including carbon fibers may require an increased shell or spar thickness to provide adequate stiffness such that the blade does not bend and strike a tower of the wind turbine. The addition of more fibers angled at between about 5° and about 35° with respect to the pitch axis to increase a thickness of the shell or spar may increase a weight of the blade and/or a cost of the blade due to the extra material used to increase the thickness. The addition of more carbon fiber, especially at an angle with respect to the pitch axis, to such blades may further increase a cost of the rotor blade because of the more expensive carbon fibers.
Swept rotor blades have also been used to reduce loads in wind turbine rotor blades. However, a torsional stiffness of swept rotor blades may resist the desired twisting/bending of the rotor blade. Some known swept rotor blades are fabricated with a reduced airfoil thickness to reduce the torsional stiffness of the blade and thereby allow the blade to twist/bend. However, reducing the thickness of the airfoil of such swept rotor blades may require more material to provide adequate strength and stiffness to withstand shear loads, thereby possibly further increasing cost and/or weight of the blade because of the extra material.